The present invention relates generally to the deposition of coatings containing cobalt oxide either by pyrolysis of coating reactants in solution or by chemical vapor deposition. The present invention relates particularly to the formation of coatings containing cobalt oxide in a mixed metal oxide spinel structure.
In U.S. Pat. No. 3,081,200, Tompkins discloses a method for preparing coatings containing oxides of metals, including cobalt, by the pyrolysis of metal acetylacetones in organic solvents. In U.S. Pat. No. 3,660,061, Donley et al disclose coating compositions comprising organic solvents and organometallic coating reactants including cobalt acetylacetonate. While no specific valence state of cobalt is disclosed as preferred, commercial activity has been developed upon the use of cobaltic acetylacetonate primarily because common cobaltous acetylacetonate, which is hydrated, is relatively insoluble in the organic solvents listed in U.S. Pat. No. 3,660,061.
In U.S. Pat. No. 3,852,098, Bloss et al disclose contacting a hot glass substrate with a gaseous mixture 50 to 100 percent saturated with the vapors at a reactive metal compound which thermally reacts to deposit a metal-containing coating on the surface. Based on thermodynamic data, from the viewpoint of vaporization behavior, cobalt III-acetylacetonates are preferred to cobalt II-acetylacetonates for chemical vapor deposition.
It has been observed that metal oxide films formed from the above-described coating compositions may include cobaltous oxide in a Co.sub.3 O.sub.4 (CoO.Co.sub.2 O.sub.3) spinel structure even though cobaltic acetylacetonate is used as the coating reactant and the film is formed in a generally oxidizing atmosphere. This observation leads to the belief that some proportion of cobaltic ions are reduced to cobaltous ions prior to the deposition of cobalt oxide.
Analysis of mixed metal oxide films formed on glass from metal acetylacetonates either by pyrolysis of coating reactants in solution or by chemical vapor deposition indicates that the metal oxides tend to assume a spinel structure which is characteristically very durable. In a spinel structure, oxygen ions form a close packed array leaving interstitial spaces to be occupied by the metal ions. The interstitial spaces having four nearest neighbors are filled by cations which coordinate tetrahedrally while the spaces having six nearest neighbors are filled with cations which coordinate octahedrally. Some metal ions will coordinate either tetrahedrally or octahedrally while others, such as chromic and cobaltic, prefer octahedral coordination. In the deposition of mixed metal oxide films, when it is desired to form a spinel structure which has a general formula of A.sub.8 B.sub.16 O.sub.32, wherein A represents cations which are tetrahedrally coordinated and B represents cations which are octahedrally coordinated, it is important to have an appropriate ratio of A and B type cations since the composition of the film may vary depending on the relative availability of A and B type cations.